(1) Field of the Invention
The invention relates to the acid treatment of subterranean earth formations, and more particularly to the acid treatment of siliceous subterranean formations surrounding oil wells, gas wells, water injection wells and similar boreholes.
(2) Description of the Prior Art
Acid treatment or "acidizing" is a well-known expedient employed for rejuvenating oil-producing and gas-producing formations and to facilitate the ease with which fluid such as water, brine or gas can be injected into subterranean formations surrounding a wellbore. Acidizing of siliceous formations, e.g. sandstone, shale, serpentines, etc., has met with some favorable results when the formation is treated with hydrogen fluoride. Various modifications of this hydrogen fluoride acidizing have been disclosed in the prior art. These modifications have mainly consisted of the use of various mixtures of hydrogen fluoride and various other mineral acids such as orthophosphoric acid, fluorophosphoric acid, sulfuric acid, hydrochloric acid, etc. Although such mixtures are generally effective, experience has shown that many formations do not respond to the acid treatment.
In general, hydrocarbon-bearing siliceous formations are of a heterogeneous nature and contain a variety of inorganic materials. In addition, the pores of the formation may contain objectional deposits of organic matter such as viscous crude oil, waxes, asphaltenes and resin precipitates of petroleum origin. Conventionally, before the acidizing treatment is begun this undesirable organic matter must be removed. Solvents such as carbon disulfide, carbon tetrachloride, or an aromatic hydrocarbon are first injected into the formation surrounding the well. This solvent treatment is repeated several times until the pores of the formation are relatively free of the organic material to insure a proper acidizing environment in the formation.
Another problem common to all methods of acidizing is the production of precipitates within the formation interstices through the action of the acid-treating reagent or its byproducts on some precipitate-forming constituent of the formation. This generally occurs when the acidizing fluid is spent and precipitates in a form which plugs the pores of the producing formation. As noted above, acidizing techniques have previously employed mixtures of phosphoric acid, generally referred to as orthophosphoric acid, with other mineral acids. However, the orthophosphates of polyvalent or heavy metals are all virtually insoluble in water. For example, calcium and magnesium compounds are found in all producing formations, and when attacked by phosphoric acid mixtures form insoluble phosphates. The calcium and magnesium phosphates are especially difficult to remove and require expensive procedures to revitalize a producing formation. Therefore, there exists a need for a composition which will provide a "one shot" acidizing treatment which removes objectionable deposits of organic material, eliminating the need for a separate solvent treatment step, along with an acidic-treating reagent that does not form precipitates within the formation.
Accordingly, a principal object of this invention is to provide a novel composition and method for increasing the permeability of siliceous subterranean formations.
Another object is to provide a composition and method which remove both acid-soluble and oil-soluble components from the formation.
A further object is to provide a composition which does not form undesirable precipitates on reaction with the formation.
Other objects, advantages and features of the invention will become apparent from the following description and appended claims.